As the septum primum grows, the ostium primum progressively narrows.

Ostium primum defects are less common than ostium secundum defects.

For a time the atria communicate with each other by an opening, the ostium primum of Born, below the free margin of the septum.

The ostium secundum provides communication between the atria after the ostium primum closes completely.

Incomplete formation of the endocardial cushions can lead to atrioventricular septal defects, such as an ostium primum defect.

The septum primum eventually fuses with the endocardial cushion, closing the ostium primum off completely.

The opening between the bottom edge of the spetum primum and endocardial cushions are the ostium primum (first opening).

Initially the atria are separated from one another by the septum primum except for a small opening in the septum, the ostium primum.

The increasingly smaller gap below it (before it fuses with the endocardial cushion) is known as the ostium primum (i.e. "the first opening").

Surgical mortality for uncomplicated ostium primum defects in experienced centers is 2%; for uncomplicated cases of complete atrioventricular canal, 4% or less.

Before the ostium primum is completely occluded, a second opening called the ostium secundum begins to form in the septum primum.

The extensions of the upper and lower endocardial pads grow along the margin of the septum primum and close the ostium primum.

Because of its crescent shape, the septum primum does not fully occlude the space between left and right atria; the opening that remains is called the ostium primum.

A defect in the ostium primum is occasionally classified as an atrial septal defect, but it is more commonly classified as an atrioventricular septal defect.

In the partial AVSD, there is a defect in the primum or inferior part of the atrial septum but no direct intraventricular communication (ostium primum defect).

In the developing heart, the atria initially communicate with each other by an opening between the free edge of the septum primum and the AV cushions, known as the primary interatrial foramen or ostium primum (interatrial foramen primum), below the free margin of the septum.